The paper argues that Rawls's account of the obligation to keep promises entails that inasmuch as we are obliged to keep promises, we are also obliged to carry out threats. On the basis of the principle of fairness, Rawls claimed that a social practice creates a moral obligation if it is just, and one has benefited from it or entered it voluntarily. A practice of threats meets Rawls's first principle of justice. We may reasonably assume that immoral threats, just like immoral promises, are not socially obliging. Threats meet Rawls's second principle of justice mainly because weaker parties benefit from drawing red lines, backed by credible threats. Finally, threats create social benefits: they contribute to social cooperation and collaboration. The argument entails that we must either accept the counterintuitive claim that threats are morally binding, or reject Rawls's account of the obligation to keep promises.